How fatherhood affects the body and brain.

Stretch Marks for Dads

What fatherhood does to the body and the brain.

Emily Anthes chatted online with readers about this article on June 14. Read the transcript here.

Marmoset baby clings to its father

Last weekend, Tufts University hosted a scientific conference on the "parental brain." Or at least the maternal brain, which was the subject of eight symposia, while fathers and their brains were the focus of just one. Once, this imbalance would have seemed inevitable, since there didn't seem to be much to say about how becoming a father affects men physically. But now, evidence is accumulating that pregnancy and parenthood leave their marks on men's bodies. Women are not the only ones who are built for parenting, and recognizing that is good for fathers and the rest of us, too.

Historically, when men did more than donate sperm to a pregnancy—by suffering physical ailments along with their wives—they got called crazy. The condition labeled "sympathetic pregnancy," or couvade syndrome (from the French word couver, or "to incubate"), describes expectant fathers who are stricken with some combination of weight gain, nausea, food cravings, backaches, insomnia, and other delights familiar to pregnant women everywhere. Until recently, couvade was relegated to the overwrought TV medical drama as a "psychosomatic" curiosity, with a list of potential causes that would please any Freudian (identification with the fetus, pregnancy envy, pseudo-sibling rivalry).

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But in the last handful of years, scientists have shown that normal, healthy, non-pregnancy-envying men often undergo real bodily changes when they're expecting children. Research shows that male marmosets and cotton-top tamarins—primates that, like humans, split child-rearing duties between the mother and father—gain as much as 20 percent of their body weight while waiting for the birth of their offspring. The finding suggests that couvade is biologically adaptive rather than psychologically neurotic: The hypothesis about the marmosets and tamarins is that the pregnancy paunch prepares a dad for the extra energy he'll expend in helping to rear his baby.

In addition, dads-to-be have elevated levels of cortisol and prolactin, hormones that are also present in high levels among mothers who are attached and responsive to their children. A father's testosterone level also drops by about a third, on average, in the first three weeks after his child is born. These hormonal shifts, which are likely sparked by exposure to the pregnant woman's hormones (there is correlational evidence that dads who spend time with moms experience the changes), mirror those experienced by mothers and may similarly prepare men for parenthood. Men who have relatively little testosterone have been shown, for instance, to hold baby dolls longer than men who are flooded with the sex hormone. High levels of testosterone, on the other hand, are associated with "incompatible non-nurturing behaviors," as one researcher put it. If dads roared along on their usual levels of the hormone, the theory goes, they'd be too busy fighting other men and seducing other women to do much diaper-changing.

There's also preliminary but tantalizing evidence that fatherhood can change the brain. A 2006 study found enhancements in the prefrontal cortex of the father marmoset. After childbirth, the neurons in this region showed greater connectivity, suggesting that having young children could boost the part of the brain responsible for planning and memory, skills parents need when having kids gives them more to keep track of. The neurons also had more receptors for vasopressin, a hormone that has been shown to prompt animal fathers to bond with offspring. (Receiving an injection of vasopressin, for instance, prompts a male prairie vole to cuddle and groom a youngster.)

And yet despite these findings, few scientists treat the physiology of fathers as a serious subject in its own right. Researchers have been investigating some of the hormonal swings in humans for almost a decade, and longer in other species; still, most of this work remains on the fringe. Between 2000 and 2006, the journal Hormones and Behavior published nearly three times as many studies of mothers as of fathers, and this year the count so far is 16 to three. A 2000 review framed research into physiological fatherhood as "an opportunity to better understand maternal behavior, by studying parental behavior in the absence of pregnancy and lactation." Interest in how men's bodies prepare themselves for fatherhood only seems to matter to the extent it sheds light on mothers. Meanwhile, the ways in which dads screwuptheirkids is a thriving area of research.

There are probably a variety of reasons for this disconnect. To begin with, couvade hasn't shed all its baggage, and that can make anything reminiscent of the syndrome seem like a subject for a psychiatrist's office rather than a biologist's lab. Also, physiological fatherhood is far from universal in the animal kingdom.In fewer than 10 percent of mammalian species are fathers actually involved in child-rearing. For years, researchers missed the hormonal changes in fathers because they looked only at male rats, which aren't natural caregivers.

Nor have the rest of us seemed eager to pay attention to the recent science about fathers. Few parenting books, even those directed at dads, take note of the findings about their hormones and brains—instead they either go for laughs or stick to counseling men about how to support their pregnant mates. In some ways, that's surprising, given that the gender boundaries of parenting have been eroding for some time now. You'd think that if we're ready for diaper-changing tables in men's restrooms, we'd also be ready to hear about men's hormonal barometers. And yet we don't seem to be. Maybe it seems too unsettling to treat the changes in expectant dads and moms as remotely equivalent.

But it would serve us all to get over that. Ignoring the physiological changes fathers undergo tends to let men off the parenting hook. Recognizing those changes, on the other hand, could make fathers feel more vital to child-rearing. For years, studies have shown (not that we really needed them to) that many men feel marginalized and anxious during pregnancy and the early weeks of their children's lives. But the nascent science of physiological fatherhood has already turned up evidence that shows that men's bodies are busy with their own preparations, even if What To Expect doesn't explain them. Men who worry that they're being thrust into fatherhood without ever learning how to parent might be (at least slightly) reassured to know that their bodies have begun to adapt. Armed with the knowledge that their hormone levels have shifted precisely so they'll be more apt to cuddle their newborns, men may feel entitled to do more of the soothing. Which can only be good for kids—and for tired moms. This Father's Day, it's time to thank dads for all their bodies do.

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